The present invention relates to refrigeration systems, and particularly (but not exclusively) to refrigeration systems for ultra-low temperature storage compartments.
The operation of conventional low temperature refrigeration systems is well known in the art, so will not be described in detail here. However to appreciate the benefits provided by the present invention it is necessary briefly to explain how such conventional systems work.
A basic refrigeration system comprises a compressor which compresses gaseous refrigerant and supplies it to a heat exchanger where the refrigerant is condensed to a liquid, giving out heat energy. This liquid refrigerant then passes through a flow restrictor into an evaporator whereat the low pressure and the expansion of the refrigerant causes the refrigerant to vaporise thereby absorbing heat. The gaseous refrigerant then passes out of the evaporator back to the compressor to begin the cycle again. An accumulator is usually provided in the return path between the evaporator and the compressor, and the accumulator collects any liquid vapour passing through the evaporator so as to prevent its entry into the compressor.
Systems intended to cool to lower temperatures often employ a dual-stage cascade system which essentially comprises two separate but co-operating refrigeration systems. One of these stages draws heat from the location to be cooled; this stage is generally termed the low-side because it operates. at a lower average temperature. The other stage is adapted to cool the compressed low-side refrigerant and release the heat to the external environment. As this second stage operates at a higher average temperature it is generally termed the high-side.
Refrigeration systems, including dual stage systems, seldom operate continuously, but instead switch as appropriate between an on-time and an off-time so as to maintain the desired temperature. This on/off switching aims to make the operation of the refrigeration system as efficient as possible, and is usually controlled thermostatically.
Many refrigeration systems, and particularly low temperature dual-stage ones, use a capillary tube as the flow restrictor (also called a metering device), and whilst such systems have many advantages, they are still far from totally efficient. The main cause of this lack of efficiency is the unnecessary time of operation when in an on-condition.
When a refrigeration system shuts off, gaseous refrigerant will condense within the evaporator, and this refrigerant will collect in the accumulator. During normal operation liquid refrigerant that has not been fully vaporized is stored in the accumulator, particularly at the end of the cycle as the load on the evaporator decreases. When a capillary tube is employed as a metering device liquid refrigerant continues to flow through the evaporator during the off cycle until the system pressure balances. If an accumulator is effectively to protect the compressor by preventing liquid refrigerant from entering it at the start of an on-time, the accumulator needs to be able to hold all of the charge of refrigerant that passes through the evaporator in the off-time. Unfortunately, when the system starts an on-time, it takes a while for the liquid refrigerant in the accumulator to vaporise and return to the compressor. When the compressor starts, the slow rate of evaporation causes the pressure at the input to the compressor to drop to a low level until a sufficient amount of vapour is available for compression, to be passed round the system. This low pressure at the start of a period of operation drastically reduces the volumetric efficiency of the compressor and prolongs the on-time.
In essence the compressor, which consumes the most energy in the cycle, operates at the beginning of an on-time at a lower efficiency. The efficiency (i.e. the amount of cooling achieved per unit of energy input) of the compressor increases up to an optimum as the on-time continues, but the delay in reaching this optimum efficiency can cause the on-time to be unnecessarily prolonged.
Therefore, according to the present invention there is provided a refrigeration system comprising a compressor, an evaporator to which refrigerant compressed by the compressor is supplied through a flow restrictor, and return means to direct back to the compressor refrigerant expanded in the evaporator, in which system the return means includes an accumulator in which may collect any liquid refrigerant leaving the evaporator, the accumulator being provided with heating means to evaporate liquid refrigerant collected by the accumulator.
In use the heating means provides sufficient heat to the liquid refrigerant in the accumulator to cause at least partial vaporisation thereof. Heating the accumulator causes the liquid refrigerant to vaporise more rapidly and at a higher pressure. This gaseous refrigerant then can return to the compressor to pass round the cycle. By encouraging the liquid refrigerant in the accumulator to vaporise, the time taken for the compressor to achieve optimum efficiency is dramatically reduced, and in consequence the length of any on-time may be significantly shortened. Tests have shown a 40%-50% reduction in the average duration of on-times.
If the accumulator were to be heated continuously the pressure would remain high, thus inhibiting the operation of the evaporator. Therefore it is highly preferred that there is provided a controller for the heating means, which controller is arranged to cause the heating means to operate for an interval at the commencement of a period of refrigeration following a period of inactivity of the system.
The duration of the heating interval may preferably be between 2 and 6 minutes, with 3-4 minutes being even more preferred. The control of the duration of the heating interval is dependent on a number of factors, such as the temperature of the refrigerant.
This system is primarily intended for use in an electrically powered refrigeration system. As such the heating means may be electrically powered. The heating means may be integrally formed with the accumulator, or may be a separate unit applied to the outside thereof. If applied separately, the heating means may be in the form of a flexible electric heating pad placed against a portion, preferably at least a lower portion, of the accumulator. Alternatively the heating means could utilise hot gas from other parts of the high or low side, flow of which could be controlled by a solenoid valve.
To ensure accurate and reliable control of the refrigeration system, the controller may comprise or include a microprocessor adapted to operate the heating means. The controller may operate in response to data pre-stored therein. In addition, or instead, the controller may operate in response to data received from external sources such as a pressure sensor, temperature sensors or other control equipment.
The present invention is applicable to various refrigeration systems, but will find a particular application to low temperature dual-stage systems. In such systems the heating means could be applied to both an accumulator on the low-side and an accumulator on the high-side. In practice, testing so far has shown that application of heating means to a high-side accumulator provides little if any increase in efficiency; thus, for reasons of economy, the heater may be provided on the low-side accumulator only.
It is normal for a conventional refrigeration system to include some form of control means, even if it is as simple as a thermostatic switch linked to the power supply. For reasons of simplicity and overall efficiency, the controller may be integrated in, or part of, an overall system controller.
In addition to the advantages discussed above, the present invention offers at least two further advantages. Firstly if the heater should fail, the system can still continued to operate as an existing system thereby not increasing the risk of damage to the goods stored in a freezer using such a refrigeration system. In effect it is fail-safe. Secondly, use of the present invention simplifies the task of charging the system with refrigerant during manufacture. In existing systems, the amount of refrigerant introduced into the system has to be carefully controlled. Too little refrigerant causes poor performance, but if too much is contained, not only can performance suffer, but more importantly refrigerant condensing during the off-time can overflow the condenser and flood the compressor. In systems according to the present invention larger capacity accumulators may be used without sacrificing performance, as would be the case in existing systems. Therefore, a greater degree of variation in the amount of refrigerant introduced can be tolerated and the process of manufacture simplified.
According to the present invention, there is also provided a low-temperature storage compartment, such as a freezer, incorporating a refrigeration system as hereinbefore described.